Five experiments with 353 young adults performing a game on an aggression machine are reported, three of them testing hypotheses concerning the effects of a model's aggressiveness and social status on instrumental aggressive behavior, and two investigating aspects of reliability and validity. Model's aggressiveness increased the Ss' aggressive behavior in all experiments. The use of presumptive monetary losses as a means by which SS punish their opponents in aggression machine experiments is as effective as presumed electric shocks, and therefore seems preferable in further experimental studies of aggression.